There are numerous circumstances in which it is desirable to detect, measure or monitor a constituent of a fluid. One of the commonest requirements is to determine hydrogen ion concentration (generally expressed on the logarithmic pH scale) of aqueous fluids which may for example be a water supply or an effluent. The determination of the pH of a solution is one of the most common analytical measurements and can be regarded as the most critical parameter in water chemistry. Nearly all water samples will have their pH tested at some point in their life cycle as many chemical processes are dependent on pH. Another common requirement is to determine oxygen content in water.
A particularly challenging context is the analysis of downhole fluids, which can be an important aspect of determining the quality and economic value of a hydrocarbon formation. Knowledge of downhole formation (produced) water chemistry can be applied to save costs and increase production at all stages of oil and gas exploration and production. Measurements obtained downhole can be important for a number of key processes of hydrocarbon production, including:                Prediction and assessment of mineral scale and corrosion;        Strategy for oil/water separation and water re-injection;        Understanding of reservoir compartmentalization/flow units;        Characterization of water break-through;        Derivation of the water cut RW; and        Evaluation of downhole H2S partition in the oil and or water (if used for H2S measurements).        
Some chemical species dissolved in water (for example, Cl− and Na+) do not change their concentration when moved to the surface either as a part of a flow through a well, or as a sample taken downhole. Consequently information about their quantities may be obtained from downhole samples and in some cases surface samples of a flow. However, the state of chemical species, such as H+ (pH=−log [concentration of H+]), CO2, or H2S may change significantly while tripping to the surface. The change occurs mainly due to a difference in temperature and pressure between downhole and surface environment. In case of samples taken downhole, this change may also happen due to degassing of a sample (seal failure), mineral precipitation in a sampling bottle, and (especially in case of H2S)—a chemical reaction with the sampling chamber. It should be stressed that pH, H2S, and CO2 are among the most critical parameters for corrosion and scale assessment. Consequently it is of considerable importance to determine their downhole values and there have been proposals for analytical sensors to be used downhole.
One approach to measurements, both at the Earth's surface and downhole, involves a solid-state probe utilising redox chemistries at the surface of an electrode.
U.S. Pat. No. 5,223,117 disclosed a pH sensor in which two molecular species were attached to a gold substrate. This was used with a very much larger counter electrode. The species attached to the gold substrate were described as effectively providing two electrodes shorted together, whose potential relative to the counter electrode could be controlled. One of the attached species was ferrocene which served as reference electrode because it displays a redox potential which is generally insensitive to the concentration of hydrogen ions. The other was hydroquinone which served as an indicator electrode, with a redox potential that is dependent on hydrogen ion concentration. This sensor was used in voltammetry in which the gold substrate with the attached redox systems and the counter electrode are placed in contact with a solution to be tested. The potential applied to the gold substrate was systematically varied and current flow was monitored. A plot of current against applied voltage, a so-called voltammogram, shows current peaks when the redox reactions take place at specific values of applied voltage. The difference in voltage between the voltages giving peak current for the ferrocene reference and a peak current for hydroquinone provides a measure of the pH of the solution under test.
WO 2005/066618 disclosed a sensor in which two different pH sensitive molecular redox systems and a pH insensitive ferrocene reference were attached to the same substrate. One pH sensitive redox system was anthraquinone (AQ) and the second was either phenanthrenequinone (PAQ) or alternatively was N,N′-diphenyl-p-phenylenediamine (DPPD). Combining the responses of the two pH sensitive redox systems relative to the reference gave an increasing sensitivity to pH.
In both of the disclosures mentioned above the redox systems were mixed together and applied to the conductive substrate. Consequently the proportions of redox systems provided on the substrate were dependent on the proportions in the mixture and the efficacy of take-up onto the substrate.
WO 2007/034131 disclosed a sensor with two redox systems incorporated into a copolymer. The exemplified copolymer was made from vinyl ferrocene and vinyl anthracene so that the two redox systems were present as side chains from the hydrocarbon backbone of the polymer. This use of a polymeric species as active component of the electrode was stated to decrease instability in the electrode performance.